This project will develop a low-noise transcranial magnetic stimulation (TMS) system. TMS is a technique for non-invasive brain stimulation using strong, brief magnetic pulses. TMS is widely used as a tool for probing brain function and is an FDA approved treatment for depression. A significant limitation of TMS, however, is that the magnetic pulse delivery is associated with a loud clicking sound as high as 140 dB resulting from electromagnetic forces. The loud noise significantly impedes both basic research and clinical applications of TMS. First, it effectively makes TMS less focal since every click activates auditory cortex, brainstem, and other connected regions, synchronously with the magnetic pulse. Second, the repetitive clicking sound, both by itself or paired with synchronous activation at the TMS target site, can induce neuromodulation that can interfere with and confound the intended effects at the TMS target. Third, the clicking noise can compromise blinding of TMS studies and necessitates the use of sham conditions that replicate the sound but that could induce undesirable sound-mediated modulation effects as well. Finally, there are known safety concerns regarding hearing loss and induction of tinnitus, especially in vulnerable populations, as well as tolerability considerations, since TMS noise may contribute to headache and cause discomfort in some patients. Addressing this need, we propose a quiet TMS (qTMS) device that incorporates two key concepts: First, the dominant frequency of the TMS pulse sound (typically 2?5 kHz) will be shifted to higher frequencies that are above the human hearing upper threshold of about 20 kHz. This will be accomplished by making the magnetic pulse ultrabrief, and shaping it so that its fundamental frequency is above 20 kHz. Due to the strength?duration properties of the neural response, ultrabrief pulses require higher amplitude to achieve neural stimulation, but the total pulse energy is actually lower than for conventional pulses. Second, the TMS coil will be redesigned electrically and mechanically to generate suprathreshold electric field pulses while minimizing the sound emitted at audible frequencies (< 20 kHz). This will require the coil to sustain pulses with higher voltage and current but of briefer duration than conventional pulses, while minimizing the electromagnetic energy that is converted to and emitted as acoustic energy at frequencies below 20 kHz. The enhanced acoustic properties of the coil will be accomplished with a novel, layered coil design. We will design and build a qTMS device based on these concepts, aiming at an initial reduction of the acoustic noise of 40 dB compared to a conventional device. The neural and acoustic stimulation produced by qTMS will be characterized in bench-top measurements and a proof-of-concept human study. We present pilot data from a low-amplitude qTMS prototype already demonstrating reduction of noise by 19 dB with ultrabrief pulses, as well as data from a human study showing comparable neural activation with amplitude-adjusted brief versus long pulses. Thus, qTMS technology could enable more precise, effective, safe, and tolerable TMS.